1. Field of the Invention
This invention relates to a radiation shield for pipes conveying a medium through a heater and more particularly to method and apparatus for controlling the rate of transfer to heating surfaces by radiation shields positioned in close proximity to an arrangement of pipes through which a medium flows and is maintained separate from the products of combustion.
2. Description of the Prior Art
It is known in conventional preheaters or radiation recuperators to convey hot flue gas through a flue chamber in heat transfer relation to a medium, such as air, to be heated. The medium to be heated in one arrangement flows through heating passages which are heated by the radiant heat from the flue gas passing through the flue chamber.
In another arrangement the medium to be heated is directed through an additional passageway which is heated by the flue gas at a point distant from the flue chamber. Accordingly, the capacity of the preheater or recuperator to heat the medium is increased by additional heat transfer sections.
In a shell-type combustion furnace recuperator a vertical inner liner receives flue gas exhausted from a furnace. An intermediate liner surrounds the inner liner to form an air heating passageway therethrough. An inlet box supplies hot flue gas to one end of the passage. A discharge means receives the waste flue gas from the other end of the passage. An outer metal support shell having a refractory lining surrounds both the inner and intermediate liners.
In certain preheater operations, it is necessary to limit the heat transfer to two or more widely different rates, for example, 12,000 BTU/Ft.sup.2 Hr for a certain of the fire box and 7,500 BTU/Ft.sup.2 Hr for another section of the fire box. Different heat transfer rates as above require different flame radiating temperatures. Because the above heat transfer rates are relatively low, low flame radiating temperatures are required, particularly if the heating passages or pipes are exposed to direct flame radiation. Low flame radiating temperatures require large volumes of flue gas recirculation with fuel fill firing or low fuel firing without any gas recirculation. Low fuel firing in large fire boxes results in poor mixing, considerable thermal gradients in the flue gas, and heat transfer variation in the pipes.
There is a need in fired heaters and the like to control the heat transfer rate to the radiant section heating surfaces while permitting the use of relatively high flame radiating temperature as opposed to relatively low flame radiating temperatures.